The present invention relates generally to electronic signal probes and more specifically to a method and apparatus for testing whether or not a probe has been calibrated.
Electronic measurement apparatus for measuring various characteristics of an input signal, including oscilloscopes, frequency counters, digital multimeters (DMMs), logic analyzers, or the like, receive input signals in various frequency bands through voltage attenuation probes. The input impedance associated therewith results from the input impedance of the electronic measurement apparatus. It is necessary to calibrate the attenuation probe so as to minimize signal waveform distortion caused by the probe when the signal is accessed by the probe. To this end, according to one method of the prior art, a calibration square-wave signal from a calibration generator is applied to the probe, and the output signal waveform from the probe is observed with the oscilloscope. When the output signal waveform is a correct square-wave, the probe is determined to be calibrated. When the output signal waveform from the probe is not the correct square-wave, the probe calibration is adjusted while observing the waveform with the oscilloscope. Since many electronic measurement devices do not have an oscilloscope function associated therewith, a separate oscilloscope is needed to check the probe calibration. As a result, probe calibration is troublesome and expensive.
Other conventional methods have been developed to determine whether or not an attenuation probe is calibrated without the use of an oscilloscope. One such conventional method of testing probe calibration is disclosed in Japanese Published Unexamined Patent Application No. 52-137954 corresponding to U.S. Pat. No. 4,070,615. According to this method, a square-wave signal is applied to the probe and to one input terminal of a differential amplifier, the output signal from the probe being applied to another input terminal of the amplifier. An output signal from the differential amplifier is then smoothed and applied to a light emitting diode (LED). An operator may ascertain that the probe is calibrated when the brightness of the LED is at a maximum. However, the method is limited in accuracy because it is difficult to make a very accurate determination of maximum brightness.
Another conventional method of testing probe calibration is disclosed in Japanese Published Unexamined Patent Application No. 55-147368, corresponding to U.S. Pat. No. 4,253,057, wherein a square-wave is applied to an under-compensated probe and a reference level is set just above the peak level of a resulting probe output signal. The probe is then adjusted toward overcompensation until the output level exceeds the reference level, as determined by a comparator having an output which drives an indicating lamp. This method has a disadvantage in that the operator must know whether the probe is in an under-compensation (under-shoot) condition or in an over-compensation (over-shoot) condition before calibration, since calibration must be started when in the under-compensation state. Another disadvantage of the method is that the operator cannot determine whether or not the probe is calibrated without first modifying it.